Author Topic: Glossary  (Read 49976 times)

waveney

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Glossary
« on: October 07, 2008, 02:41:16 pm »
Galaxy Zoo Forum Glossary Produced by Geoff Roynon, Pat and waveney

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Links to other Glossary pages

Lawrence Berkley National Laboratory
Stardate Online
Australia Telescope Outreach and Education
Caltech Glossary
SDSS Glossary
University of Chicago
University of Virginia
University of Illinois

See Also:

List of cosmology links


Additional contributions from: Alice, dthomas02, EigenState, FermatsBrother, Infinity
« Last Edit: April 03, 2009, 08:43:49 am by waveney »
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Glossary A
« Reply #1 on: October 07, 2008, 02:41:48 pm »
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Absolute Magnitude 
How bright a star would look if it were 32.6 light years away from the Earth.

A star's absolute magnitude is the magnitude the star would have if it were placed at a distance, from us, of 10 parsecs, which is about 33 light-years.  This is a derived number, because you need an estimate of a star's distance to convert its apparent magnitude to an absolute magnitude.  And a star's apparent magnitude comes from measuring its brightness by eye, or with a photometer, or some other instrument, and correcting for airmass.[Nereid]
Links:
Wikipedia link
Saturday 9th August 2008 OOTD post

Absolute Zero
The lowest possible temperature -273.15 °C.
Links:
Wikipedia link

Absorption Line
Dark lines in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object. Most stellar spectra, including that from our Sun, are absorption spectra.
See “Spectral Lines” for links.

AGN - Active Galactic Nucleus
Examples of an AGN are Seyfert 1 or 2 galaxies or elliptical galaxies with an active galactic nucleus, such as M87 in Virgo. Even though all Quasars (QSO), Blazars or the Blazar subtype, BL Lac objects all have active galactic nuclei, not all galaxies with active galactic nuclei are Quasars (QSO), Blazars or the Blazar subtype, BL Lac objects. Actually, quasars are only one of many subtypes of AGN. 'AGN' encompasses all galaxies that have active galactic nuclei. [Eric Diaz]
Links: AGN Thread
        AGN info from Bill Keel
        OOTD entry on Black Holes and AGN

AGN Unification models
These summarise all AGN types into a single picture of galaxy activity and evolution - with the observed properties depending on the intrinsic luminosity and the orientation relative to our line of sight.
Wikipedia link

Angstrom
A unit of length equal to one hundred-millionth (10-8) of a centimeter, used especially to specify radiation wavelengths.
Links:
Wikipedia Link
Fun With Units

Arc Seconds, Arc Minutes and Degrees for coordinates
There are 60 arc seconds to an arc minute and 60 arc minutes to a degree and 360 degrees to a complete circle. These are used for angular measurements of objects in astronomy.

The sun and moon both appear about 30 arcminutes in diameter from Earth; Venus at its closest appears just about one arcminute in diameter. At a distance D, one arcminute spans a linear distance (perpendicular to the line of sight) of D/3438. (One arcsecond spans D/206,254, a familiar astronomical constant).
Links:
Wikipedia link to Arcseconds
Basic Measures in Astronomy

Asterism
An Asterism is a star pattern that isn't part of a constellation.
Links:
Wikapedia Link on Asterism
List of Asterisms

Asteroid
Asteroids are rocky bodies that are smaller than planets that orbit stars. Also known as minor planets. They show up in SDSS photos mainly as blue red green or green red blue blobs. They are fast moving objects so will travel between the different coloured filters of the SDSS camera.
Links:
Sloan Digital Sky Survey Link on Asteroids
Bill Arnett's Nineplanets website on asteroids
Wikapedia's link on Asteroids
Link: Inner Solar System Asteroids



« Last Edit: May 17, 2009, 05:37:59 pm by waveney »
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Glossary B
« Reply #2 on: October 07, 2008, 02:42:15 pm »
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Barred Spirals
Barred Spirals have a straight bar at the middle and then usually well defined arms from the ends of the bar.  The Milkyway is a barred galaxy.

Bias Study
The original Galaxy Zoo classification project found that people were reporting more anti-clockwise spirals than clockwise spirals which did not fit with the way the astronomers thought the universe should work. So the zookeepers designed a project to test if the people doing the classification had a bias towards reporting spirals as anti-clockwise when the rotation wasn’t clear. Part of this project involves displaying the images of galaxies rotated in various ways.
This is why some galaxies appear different when you look at them on the SDSS site.
You must always classify what you see on the Galaxy Analysis page before looking at the image in SDSS. See the link below for more information.
Link: Forum Thread - Bias Study

Binary Stars
Binary stars consist of two stars orbiting each other around their common centre of mass. Binary star systems are very important in astrophysics as their masses can be calculated from observing their orbits. If binary stars happen to orbit in a plane to our line-of-sight they will eclipse each other and are called eclipsing binaries.
Links: Wikipedia - Binary star
          Cornell - Binary Stars
          Forum Thread - Binaries

BL Lacertae objects / BL Lac objects
These are blazar subtype of AGN - an intrinsically weak radio galaxies. They are characterized by a relativistic jet aligned towards us, which can fluctuate much more rapidly than the gigantic host galaxy. It is this rapid dramatic change and optical polarisation that we see - leading to the early classification of the prototype object as a variable star. Compared with the distinct and pronounced peaks of quasar spectra,  BL Lac object spectra are relatively featureless.

Glossary links: AGN's, Blazar, Quasar, Jet

Wiki link: BL Lac objects, blazar, superluminal motion

Black Holes
Black holes are regions of space where gravity is so strong that nothing, not even electromagnetic radiation, can escape - hence the term “Black Hole“. They have only three properties - mass, angular momentum and charge.

We observe them through the behavior and emissions of matter caught in their gravitational field. Periods of high accretion - "feeding" - give rise to the enormous energy outputs of AGN's and QSO's.

Supermassive black holes mass millions or billions of suns and form the cores of galaxies.
Stellar Mass black holes massing 1.5 to 20 suns formed in Supernovae or from the merging of binary neutron stars.
The existence of Micro and intermediate size black holes is less well supported.

Links: NASA's Imagine The Universe link on Black Holes
Hubblesite Black Holes
About.com:Physics
Wikipedia - Black holes
OOTD entry on Black Holes and AGN
OOTD entry on the size of Black Holes

Blazar
A very compact and highly variable energy source associated with a supermassive black hole. Blazars are AGN with a relativistic jet that is pointing in the general direction of the Earth. [Wikipedia]
See links under “AGN” for more info.
Link: Blazar description in Wikipedia

Blueshift
Any object approaching us (i.e. with negative z).  See Redshift for more information.
Link: Blueshift OOTD

« Last Edit: October 21, 2009, 02:03:00 pm by waveney »
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Glossary C
« Reply #3 on: October 07, 2008, 02:42:37 pm »
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Comets
A comet is a body that orbits the Sun and shows a tail when close to the Sun. Its nuclei is thought to consist of a collection of ice, dust and rocks. Comets are thought to originate in the Oort Cloud and/or the Kuiper Belt.
Links: Wikipedia - Comet
          Interactive Comet Animation
          Forum Thread - Comets
          OOTD on Comets

Conversion of RA & DEC
RA is measured in hours, minutes and seconds and can be stated as 8h 11m 43s or 08:11:43.87.
DEC is measured in degrees, arcminutes and arcseconds and can be stated as +15d 32’ 8” or +15:32:08.
On the SDSS Skyserver Object Explorer page (and elsewhere within SDSS) the RA and DEC values are given in degrees only, e.g. ra=130.43319704, dec=13.06579643
An online Convertion Tool

It is sometimes useful to be able to convert the SDSS values into the usual units for use elsewhere. If you already have an object from SDSS then use method (1) as this is the simplest.

(1) The information is at the top of the thumbnail image in the "Skyserver Object Explorer" page.

see: http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587742061050003470

ra=130.43319704, dec=13.06579643

Beside thumbnail is: SDSS J084143.96+130356.8

This translates to ra = 08:41:43.96 dec = +13:03:56.8

(2) On a calculator:

If you have ra=130.43319704, dec=13.06579643

For RA.
Enter 130.43319 (5 decimal places is OK)
Divide by 15 gives 08.6955467
The "08" is your Hours

Subtract 08 gives 0.6955467
Times 60 gives 41.7328
The "41" is your minutes

Subtract 41 gives 0.7328
Times 60 gives 43.968....
The "43.9" is your seconds

So RA = 08:41:44

For DEC.
Enter 13.06579 (5 decimal places is OK)
The "13"  is your degrees

Subtract 13 gives 0.06579
Times 60 gives 03.9474
The "03" is your minutes

Subtract 03 gives 0.9474
Times 60 gives 56.84...
The "56.8" is your seconds

So DEC = +13:03:57

If the given dec value is negative, the final value is also negative.

[Thanks to Fermats Brother]

Cosmic Rays
Cosmic rays are energetic particles originating from space. They are not "rays" at all but discrete particles. About 90% are hydrogen (protons), about 9% are helium. All of the other elements are present but constitute only about 1%.

Cosmic ray detectors can measure slight differences in the energy and elemental and isotopic composition which indicate the source. Known sources include the Heliosheath (very low energy), the sun (low energy), Galactic and extra-galactic which are primarily supernovae (high energy) and ultra high energy cosmic rays - believed to come from AGN’s and Gamma Ray Bursts. The energy depends upon the strength of the magnetic field that accelerated them.

They can affect light detectors and photographic plates and may show up on SDSS images

Links:Wikipedia link on Cosmic Ray
Wikipedia link on Ultra-high-energy cosmic ray
NASA's Imagine the Universe link on Cosmic Ray


« Last Edit: June 12, 2009, 12:08:56 pm by waveney »
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Glossary D
« Reply #4 on: October 07, 2008, 02:42:54 pm »
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Dark Matter
This is a type of matter that cannot be seen and only interacts gravitationally with visible (Baryonic) matter. It makes up over 20% of the universe, as opposed to less than 5% baryonic matter. Its most obvious affects -  galaxy rotation rates, galaxy cluster dynamics and gravitational lensing - indicate a  uniform spherical Dark Matter halo around galaxies extending beyond the visible. Models and simulations allowing for dark matter best explain the current observed state and structure of the Universe."

Sounds a bit esoteric - but there are implications for SSDS users throught effects on gravitational lensing and galactic interaction - which may occur well before the visible elements of the galaxy touch, especially if the contact is outside of the galactic plain.

Link:Wikipedia - Dark Matter

Dating SDSS Images
Sometimes it is useful to know the date that an SDSS image was created, for example, to check whether the current image could contain a supernova which exploded in a particular year. This procedure should work for all images.
From the SkyServer Object Explorer page for the object in question, click on the “Field” link on the left under the “PhotoObj” heading. Scroll down to the “mjd_u” label and note down the value in the right-hand field. This is in MJD format (Modified Julian Date) and in scientific notation. Convert the value to standard decimal and add 2400000.5 to the value. As an example, if the original value is “5.26674166E4” it would become “52667.4166” then “2452667.9166”. Now enter this value into the “Julian Date” field on the following web site and click the button to get your date.
Links: Julian Date Converter

DEC - Declination
See “RA and DEC”

Deflector
A deflector refers to the large mass object (in gravitational lensing) that deflects the light of a source galaxy or quasar to produce an image to the observer of the source galaxy or quasar.
Links:
Peter Newbury's research page on Gravitational Lensing

Density Wave
The Density Wave theory is used to explain the spiral structure of spiral galaxies.
See the following links for a complete description.
Links: Wikipedia - Density Wave Theory
          Astrophysics Spectator Article
          Forum Thread - Stochastic Self-Propogative Star Formation

Dust
Interstellar dust consists of dust grains which are mainly composed of graphite, silicates and polycyclic aromatic hydrocarbons. Dust is one of the banes of astronomers as it interferes with the light from distant objects by scattering and/or absorbing it.
Links:  Berkeley - Dust Maps
           Bill Keel - Dust

Dust Lanes
Dust lanes show up as dark areas across the width of edge-on galaxies. They can also be seen in ellipticals that have recently merged with a spiral galaxy.
Links: Washington - Dust Lanes
          APOD - Sombrero

Dwarf Galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars. They come in many different morphologies:
  Dwarf elliptical galaxy (dE)
  Dwarf spheroidal galaxy (dSph)
  Dwarf irregular galaxy (dl)
  Dwarf spiral galaxy
Link: Wikipedia Article


« Last Edit: January 05, 2009, 07:50:23 pm by waveney »
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Glossary E
« Reply #5 on: October 07, 2008, 02:43:26 pm »
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Eddington limit (Eddington luminosity)
The upper brightness of an object depending on its mass - a balance of radiation pressure and gravity. Radiation pressure from gas, heating up as it falls in to a quasar, will reduce the gas flow - a feedback mechanism keeping the luminosity of the quasar below the Eddington limit.

Cataclysmic objects such as GRB's, nova and supernova my exceed the limit for a short period.
Only Accretion powered sources such as X-ray binaries and AGN's can remain close to the Eddington limit for long periods.
Wikipedia link to Eddington Limit

Einstein Cross
A lens that forms a cross by having the image that is lensed forming the cross around the central foreground galaxy.
Links:
Wikipedia's link on the Einstein Cross 
APOD Photo of Einstein Cross

Einstein Ring
A lens that is formed when light from a bright galaxy, star or Quasar is bent around an object that has a large mass such as a black hole or a galaxy or cluster of galaxies. This is a form of Strong lensing in the shape of a ring or part of a ring.
Links:
Wikipedia link 
Discovery of the First "Einstein Ring"

Emission Line
Bright lines produced in a spectrum by a luminous source, such as a star or a bright nebula. Emission lines are produced by many astrophysical processes such as those found in emission nebulae (including planetary nebulae and HII regions), quasars and some stars such as Wolf-Rayets.
See “Spectral Lines” for links.

Expansion of Space
The following links, supplied by Edd, may help if you’re grappling with the question “Does space expand?”
Links: Does Space Expand?
          A diatribe on expanding space
          The kinematic origin of the cosmological redshift

Extinction
The attenuation of starlight due to absorption and scattering by interstellar dust.
Links: Wikipedia - Extinction

« Last Edit: December 08, 2008, 04:47:05 pm by waveney »
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Glossary F
« Reply #6 on: October 07, 2008, 02:43:46 pm »
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FITS Files (Flexible Image Transport System)
FITS is a standard method for storing astronomical data and is used for the transport, analysis, and archival storage of scientific data sets. FITS is the most commonly used digital file format used in astronomy.
See the forum link below for a tutorial on using the FITS file to look at SDSS images.
Links: FITS Home Page
          Wikipedia FITS Entry
Forum Link: Pat’s Tutorial on using FITS

Flocculent Galaxy
Flocculent spirals have a swirling pattern with lots of small short arm segments, rather than clearly defined arms.

« Last Edit: April 06, 2009, 03:51:39 pm by waveney »
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Glossary G
« Reply #7 on: October 07, 2008, 02:44:29 pm »
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Galaxy Types
The main galaxy types of interest to Galaxy Zoo (phase I) are Ellipticals and Spirals. These can be further sub-divided using different classification schemes, such as “Sa”, “Sb” etc.
See “Hubble Sequence”.
Links: Bill Keel’s Galaxy Classification

Globular Cluster
A globular cluster is a collection of thousands of stars which are gravitationally bound together and form a spherical shape. They are found in the halo of a galaxy and the Milky Way has about 150.
Links: Wikipedia - Globular Cluster
          Forum Thread - Star Clusters Index
          Forum Thread - Globular Clusters

Gravitational Lensing
Multiple images of a distant object (galaxy, quasar, etc) are sometimes created if there is a massive galaxy (or cluster of galaxies) in the line of sight to the distant object. The gravitational field of the massive object bends the light from the distant object and thus acts like a lens.
Links: Gravitational Lensing
   Forum Thread - OOTD 7 Feb 2009

Gunn-Peterson trough
At red shifts > 6 the absorption lines in the Lyman alpha forest overlap to form a broad absorption trough. This demonstrates a prevalence of opaque neutral hydrogen at z>6 before re-ionisation ended the dark ages. This was predicted theoretically before SDSS data identified the first Quasar with z>6 - finally allowing the trough to be observed.
Glossary links - Lyman alpha forest, Quasar, red shift
Wiki link - Gunn-Peterson trough, re-ionisation

« Last Edit: February 18, 2009, 10:40:11 am by waveney »
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Glossary H
« Reply #8 on: October 07, 2008, 02:44:52 pm »
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Hanny's Voorwerp
Probably a Quasar light echo.
Link: for links to descriptions, science, press and everything

Herbig-Haro Object
These are patches of nebulosity associated with new stars and are formed by gas emitted by new stars colliding with nearby clouds of gas and dust.  These objects are very short-lived, lasting only for a few thousand years.
Link: Wikipedia - Herbig-Haro Object
       OOTD - 14 Aug 2009

H-R Diagram (Hertzsprung-Russell Diagram)
The basic Hertzsprung-Russell (H-R) diagram is a graph with the horizontal scale showing the star’s surface temperature and the vertical scale showing the star’s luminosity. When many stars are plotted on this graph, the majority of them fall on a diagonal running from hot, bright (top left) to cool, dim (bottom right), which is called the “Main Sequence”.
Most stars spend much of their lifetime on the main sequence while they are burning hydrogen to helium through nuclear reactions.
The Wiki link below has a nice colour diagram.

Links: Wikipedia - Hertzsprunr-Russell Diagram
   HR Diagram Simulator

HII Region
These are star forming regions where hydrogen clouds are ionised by intense ultra violet radiation from hot young stars within. They are named after the spectral emission line for ionized hydrogen. HII regions look pink in visible light photographs, but not SDSS which uses other filters.
Wikipedia link to HII Region

Hubble Sequence
The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble. His scheme divides galaxies into three broad classes based on their visual appearance:
  Elliptical (E0 - E7)
  Spiral (Sa, Sb, Sc, SB)
  Lenticular (S0
Link: Hubble Sequence

Hyper-Velocity Stars
There are 17 confirmed Hyper-Velocity Stars (HVSs) and the reason for their speed is in theory that they were binaries and got pulled by the black hole we have in the center of the Milky Way and one of them was caught on the gravitational pull and the other one went ballistic with enough velocity to be able to escape the Milky Way in a few million years.

« Last Edit: August 19, 2009, 07:00:51 pm by waveney »
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Glossary I
« Reply #9 on: October 07, 2008, 02:45:20 pm »
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Image Dating
To calculate the date when a specific SDSS image was created see “Dating SDSS Images”.


Image Manipulation
For information on manipulating SDSS images see the FITS Files entry.

Irregular Galaxy
An irregular galaxy is a galaxy that does not fall into any of the regular classes of the Hubble sequence. These are galaxies that feature neither spiral nor elliptical morphology. They are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies.

« Last Edit: April 06, 2009, 03:53:19 pm by waveney »
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« Reply #10 on: October 07, 2008, 02:45:57 pm »
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Jets
Some galaxies containing an AGN also produce narrow beams of energetic particles and magnetic fields which are ejected in opposite directions from the plane of the blackhole’s accretion disk. These are called jets and are also known as radio jets.
Link: Active Galaxies and Quasars


« Last Edit: October 07, 2008, 03:32:37 pm by waveney »
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« Reply #11 on: October 07, 2008, 02:46:22 pm »
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Kelvin Scale
Physicists measure temperature on the Kelvin scale which has units named kelvins (K). Zero K (0 K) is absolute zero, the theoretical absence of all thermal energy. The average temperature of the universe is about 3 K. The kelvin was named after the British physicist Willian Thomson, 1st Baron Kelvin.
Links: Wikipedia - Kelvin

Kilo Parsec
1000 Parsecs, which is a measurement of length, that is about 3,260 light years.
Links:
Wikipedia link on Parsec


« Last Edit: December 03, 2008, 09:22:34 pm by waveney »
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« Reply #12 on: October 07, 2008, 02:46:45 pm »
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Lensing
Lensing occurs when a massive object or collection of objects acts like a lens allowing more distant objects to be seen.
See: Einstein Cross, Einstein Ring, Gravitational Lensing, Microlensing, Strong Lensing, Weak Lensing.

Light Echo
A light echo is produced when the light from a bright object (quasar, nova, etc.) is reflected from interstellar dust and arrives at an observer after the initial light from the object.
Hanny’s Voorwerp is a good example of a light echo.
Links: APOD - Light Echo
     Wikipedia - Light Echo

LINER (low-ionization nuclear emission-line region) Galaxies
These are galaxies with nuclear spectra dominated by weakly ionised atoms such as O+, N+ -  atoms missing relatively few electrons - rather than strongly ionised atoms such as O++, H+ etc. The energy source driving this ionisation - either hot young stars or an AGN - has been hotly debated.

Wiki: low-ionization nuclear emission-line region (LINER)

LSB-Low Surface Brightness
Are most usually dwarf galaxies whose contrast is about 1 magnitude lower when viewed from the earth than the ambient night sky.
Links:
Wikipedia link
What are these mysterious Low Surface Brightness galaxies?
Low Surface Brightness Galaxy Gallery
Forum Thread on LSB

Lyman Alpha Forest
Light travelling from a distant galaxy is progressively red shifted so that intervening hydrogen clouds absorb the Lyman alpha line of neutral hydrogen at discrete but increasing wavelengths - giving rise to the absorption “forest” often seen in Quasar spectra below the Lyman alpha line .
Glossary links - Gunn-Peterson trough, Quasar, red shift
Wiki link - Lyman Alpha forest, Lyman Series

« Last Edit: February 18, 2009, 10:44:50 am by waveney »
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« Reply #13 on: October 07, 2008, 02:47:05 pm »
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Markarian
The Markarian catalog was produced by Benyamin Markarian and colleagues... at the Byurakan Observatory in Armenia. It included objects which were unusually bright in the near-ultraviolet, as seen in objective-prism photographs from their Schmidt telescope. Thus the catalog of 1500 objects includes Seyfert galaxies (the first large sample of these), starbursts, star-forming knots in bright galaxies, and BL Lacertae objects. I have a reformatted version of the final catalog provided by once-Soviet colleagues here. Since the Byurakan astronomers continued similar work with a different photographic setup in the Second Byrurkan Survey or SBS, the original Markarian project is now sometimes known as the FBS. ---NGC3314
Links:
Colour Indices of The Markarian Galaxies
Galaxy Zoo forum posts about Markarians
SAO Markarian Galaxies

Messier Objects
The Messier objects were collected by the French astronomer Charles Messier to help him in his hunt for comets (his main interest). He compiled a list of objects which were fuzzy and could be mistaken for comets but weren’t and this catalogue is now known as the Messier catalogue and contains 110 objects. Only objects from the Northern Hemisphere are included.
Link:  Wikipedia - Messier object

Metallicity
In Astronomy only, any element other than hydrogen or helium is referred to a metal - and stellar populations are defined according to their Metallicity.
Wikipedia Link to Metallicity
Cross reference link to Stellar Population

Microlensing
Usually not seen as multiple images or distortions instead can be seen as the source being brighter than it really is.
Links:
Berkeley - Gravitational Lensing
Wikipedia link on Gravitational Lensing

Muffins
Galaxy Zoo name for artifacts with strips of colour usually across a star, where the plates have not been aligned properly
Link: How Muffins Got Their Name

« Last Edit: November 04, 2008, 01:45:53 pm by waveney »
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waveney

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« Reply #14 on: October 07, 2008, 02:47:25 pm »
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Nebula
A nebula is an interstellar cloud composed of dust, gas and plasma. If there are stars nearby then the gas and dust will be visible due to reflection or excitation due to radiation. Some nebulae are dark, an example is the Horsehead Nebula.
Also see “Planetary Nebula”.
Links: Wikipedia - Nebula
          Forum Thread - The Nebula Collection

NED - NASA/IPAC Extragalactic Database
A database of objects outside the Milky Way galaxy.
There is a link to the NED database on the SDSS Object Explorer page.
Link: NED Introduction

New General Catalogue (NGC)
A catalogue system for deep sky objects, which contains nearly 8,000 objects. Originally compiled in the 1880s by JLE Dreyer based mainly on observations from John Hershel. It has been updated and expanded since that time.
Links: Wikipedia - New General Catalogue
          The NGC/IC Project
« Last Edit: October 09, 2008, 01:16:22 pm by waveney »
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